The purpose of this study is to conduct a model study to differentiate the enzyme variants in mice by simple biochemical assays. Therefore, several variants and isozymes in two inbred strains of mice were purified, and experimental conditions to differentiate these variants or isozymes were established. These include thermostability, urea and iodoacetate inactivation, and kinetic studies. From these studies, the electrophoretic variants of 3-phosphoglyycerate kinase-2 and the isozymes of Isocitrate dehydrogenase could be differentiated by their relative ther mostability. From this model study we think that relative thermostability of enzyme variants could be a useful tool for the detection of mutant enzymes in mutagen-treated mouse population.